The goal of plant breeding is to combine in a single variety/hybrid various desirable traits of the parental lines. For field crops, these traits may include resistance to diseases and insects, tolerance to heat, drought and salt, reducing the time to crop maturity, greater yield and yield stability and better agronomic quality. With mechanical harvesting of many crops, uniformity of plant characteristics such as germination and stand establishment, growth rate, maturity, plant height and fruit size, is important.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinating if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two homozygous plants from differing backgrounds or two homozygous lines produce a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants each heterozygous at a number of gene loci will produce a population of hybrid plants that differ genetically and will not be uniform.
Sorghum plants (Sorghum bicolor (L.) Moench) are bred in most cases by self pollination techniques. With the incorporation of male sterility (either genetic or cytoplasmic) cross pollination breeding techniques can also be utilized. Sorghum has a perfect flower with both male and female parts in the same flower located in the panicle. The flowers are usually in pairs on the panicle branches. Natural pollination occurs in sorghum when anthers (male flowers) open and pollen falls onto receptive stigma (female flowers). Because of the close proximity of male (anthers) and female (stigma) in the panicle, self pollination is very high (average 94%). Cross pollination may occur when wind or convection currents move pollen from the anthers of one plant to receptive stigma on another plant. Cross pollination is greatly enhanced with incorporation of male sterility which renders male flowers nonviable without affecting the female flowers. Successful pollination in the case of male sterile flowers requires cross pollination.
The development of sorghum hybrids requires the development of homozygous inbred lines, the crossing of these lines, and the evaluation of the crosses. Pedigree breeding methods, and to a lesser extent population breeding methods, are used to develop inbred lines from breeding populations. Breeding programs combine desirable traits from two or more inbred lines into breeding pools from which new inbred lines are developed by selfing and selection of desired phenotypes. The new inbreds are crossed with other inbred lines and the hybrids from these crosses are evaluated to determine which have commercial potential.
Pedigree breeding starts with the crossing of two genotypes, each of which may have one or more desirable characteristics that is lacking in the other or which complement the other. If the two original parents do not provide all of the desired characteristics, other sources can be included in the breeding population. In the pedigree method, superior plants are selfed and selected in successive generations. In the succeeding generations the heterozygous condition gives way to homogeneous lines as a result of self-pollination and selection. Typically, in the pedigree method of breeding five or more generations of selfing and selection is practiced. F.sub.1 to F.sub.2 ; F.sub.2 to F.sub.3 ; F.sub.3 to F.sub.4, F.sub.4 to F.sub.5, etc.
Backcrossing can be used to improve an inbred line. Backcrossing transfers a specific desirable trait from one inbred or source to an inbred that lacks that trait. This can be accomplished for example by first crossing a superior inbred (A) (recurrent parent) to a donor inbred (non-recurrent parent), which carries the appropriate genes(s) for the trait in question. The progeny of this cross is then mated back to the superior recurrent parent (A) followed by selection in the resultant progeny for the desired trait to be transferred from the non-recurrent parent. After five or more backcross generations with selection for the desired trait, the progeny will be heterozygous for loci controlling the characteristic being transferred, but will be like the superior parent for most or almost all other genes. The last backcross generation would be selfed to give pure breeding progeny for the gene(s) being transferred.
A hybrid sorghum variety is the cross of two inbred lines, each of which may have one or more desirable characteristics lacked by the other or which complement the other. The hybrid progeny of the first generation is designated F.sub.1. In the development of hybrids only the F.sub.1 hybrid plants are sought. The T.sub.1 F hybrid is more vigorous than its inbred parents. This hybrid vigor, or heterosis, can be manifested in many ways, including increased vegetative growth and increased yield.
The development of a hybrid sorghum variety involves five steps: (1) the formation of "restorer" and "non-restorer" germplasm pools; (2) the selection of superior plants from various "restorer" and "non-restorer" germplasm pools; (3) the selfing of the superior plants for several generations to produce a series of inbred lines, which although different from each other, each breed true and are highly uniform; (4) the conversion of inbred lines classified as non-restorers to cytoplasmic male sterile (CMS) forms, and (5) crossing the selected cytoplasmic male sterile (CMS) inbred lines with selected fertile inbred lines (restorer lines) to produce the hybrid progeny (F.sub.1).
Because sorghum is normally a self pollinated plant and because both male and female flowers are in the same panicle, large numbers of hybrid seed can only be produced by using cytoplasmic male sterile (CMS) inbreds. Flowers of the CMS inbred are fertilized with pollen from a male fertile inbred carrying genes which restore male fertility in the hybrid (F.sub.1) plants. An important consequence of the homozygosity and homogeneity of the inbred lines is that the hybrid between any two inbreds will always be the same. Once the inbreds that give the best hybrid have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred parent is maintained.
A single cross hybrid is produced when two inbred lines are crossed to produce the .sub.1 progeny. Much of the hybrid vigor exhibited by F.sub.1 hybrids is lost in the next generation (F.sub.2). Consequently, seed from hybrid varieties is not used for planting stock.
Hybrid grain sorghum can be produced using wind to move the pollen. Alternating strips of the cytoplasmic male sterile inbred (female) and the male fertile inbred (male) are planted in the same field. Wind moves the pollen shed by the male inbred to receptive stigma on the female. Providing that there is sufficient isolation from sources of foreign sorghum pollen, the stigma of the male sterile inbred (female) will be fertilized only with pollen from the male fertile inbred (male). The resulting seed, born on the male sterile (female) plants is therefore hybrid and will form hybrid plants that have full fertility restored.
Grain sorghum is an important and valuable food and feed grain crop. In addition, its vegetative parts are used for forage, syrup and shelter Thus, a continuing goal of plant breeders is to develop stable high yielding sorghum hybrids that are agronomically sound. The reasons for this goal are obvious to maximize the amount of grain produced on the land used and to supply food for both animals and humans.